American Express president Ed Gilligan dies at 55

American Express President Ed Gilligan died Friday morning after becoming seriously ill on a flight to New York, the company said.

Gilligan, who was 55, spent his entire 35-year career with American Express(AXP), starting as an intern and rising to one of the highest executive posts at the global bank. He is survived by his wife and four children.

"This is deeply painful and frankly unimaginable for all of us who had the great fortune to work with Ed, and benefit from his insights, leadership and enthusiasm," said Amex chairman and CEO Ken Chenault, who announced Gilligan's death in a letter to employees.

Chenault called Gilligan an "entrepreneurial spirit that has helped continually transform [American Express] through the years."

Gilligan joined the company in 1980 and worked his way up the ladder. He was named a vice president and the head of the company's business-to-business relations group in 2007. He also headed the company's business travel unit and then the international consumer card business and was appointed president in April 2013.

Foursquare co-founder Dennis Crowley, who worked with Amex executives on a partnership between the two companies, tweeted his reaction to the news.

"Super sad to learn about Ed Gilligan at AmEx. I got to know him thru 4SQ partnership. He was such a great person," Crowley tweeted.

Analysts at Keefe, Bruyette & Woods wrote in a memo to the firm's clients Friday that they saw Gilligan as being a likely successor to higher positions at American Express and "his absence obviously hurts," though the company has a "solid bench of executives."

An American Express spokesperson didn't have information about where the flight originated but she said other company executives were on the plane and it made an emergency landing before its scheduled New York arrival.

Chenault's letter said that more information would be available to employees next week.